The invention relates to an interframe coding system for transmission of a signal, such as a television signal, which has a relatively high frame-to-frame correlation, by compressing the bandwidth of the signal.
An interframe coding system represents a scheme for reduction of the transmission bit rate. In this scheme, a difference between the signals of successive frames is taken to provide a significant difference signal wherever the difference exceeds a given threshold value, which significant difference signal alone is encoded and fed to a buffer memory for transmission at a uniform bit rate. By way of example, an input video signal may be digitized in a 8 bit PCM code, for example. The digitized signal is supplied to a frame subtracter in which a difference between the supplied signal value and a picture element value from a corresponding position of a preceding frame stored in a frame memory is taken, and the resulting difference signal is fed to a threshold circuit in which a signal in excess of a given level is derived as a significant difference signal for conversion into a 4 bit code, for example, in a quantizer. The code obtained is sequentially written into the buffer memory, and thence transmitted at a uniform bit rate as a transmission code. On the other hand, the output code from the quantizer is converted into a 8 bit code of a level representative of a quantization range in which it existed, a sign bit being added to the 8 bit code. The converted code and the output of the frame memory are added together and written into the frame memory. With such interframe coding system, the number of transmission codes is reduced and the transmission bandwidth considerably compressed where a variation in the video signal is low. However, if a rapid change occurs in the picture, the frequency of occurrence of the significant difference signal will be appreciably increased, which may result in an overflow from the buffer memory. The use of a buffer memory having sufficient capacity to prevent such an overflow results in not only a high cost, but also in an awkward situation in the application of the visual telephone, for example, as a result of a delay of the picture signal relative to the voice signal. Consequently, the capacity of the buffer memory is constrained to two to three frames at most, with consequent likelihood of an overflow from the buffer memory which results in a loss of the information to give a considerable degradation in the image quality for a rapidly varying picture.
An improvement which minimizes the occurrence of such an overflow is known as FRODEC system which is described, for example, in an article "Transmitting Television as Clusters of Frame-to-Frame Differences" by J. C. Candy et al in B.S.T.J., Vol. 50, No. 6 (1971 July-August issue), pages 1889 to 1917. According to this system, the occupancy of the buffer memory is monitored, and the threshold value of the threshold circuit to which the output from the frame subtracter is supplied is controlled in accordance with the occupancy. Thus, as the buffer memory is approaching its full capacity, the threshold value is increased to reduce the generation of the significant difference signal and hence the number of codes which are written into the buffer memory. However, an excessively high threshold value causes a stepwise change in the picture signal, degrading the image quality. As a consequence, the threshold value cannot be controlled over a broad range, so that the likelihood of an overflow occurring remains for a rapidly varying picture. As further alternatives, a subsample mode is proposed in which every other picture element rather than every picture element is extracted for a rapidly varying picture, as well as the limitation of the information content by changing the operation mode to an alternate line mode in which the encoding takes place for every other scanning line. However, the image quality is degraded with these operation modes, and the frequency of their occurrence is high for a rapidly varying picture, so that the overall image quality is degraded.
In the interframe coding system, only the significant difference signal is encoded for transmission as mentioned previously. Consequently, the received signal is decoded on the receiving side to produce the significant difference signal, which is added to a corresponding picture element value of a preceding frame which is read out from the frame memory, and the sum is written into the frame memory, thus obtaining the decoded signal. The decoded digital signal then is converted into an analog signal to reproduce the original video signal. In view of the sequential formation of the sum of the received decoded signal value and the signal value from the frame memory on the receiving side, any code error which might occur over the transmission channel may be directly written into the frame memory on the receiving side, thus preserving such error. In consideration of this drawback, the 8 bit PCM codes from several horizontal scanning lines per each frame are directly supplied, after A-D conversion, to the buffer memory, and on the receiving side, the received PCM codes are directly written into the frame memory without being added with the corresponding code from the frame memory. The horizontal scanning lines for which the PCM codes are directly supplied are shifted for successive frames so that the frame memory on the receiving side can be refreshed over a plurality of frames. Conversely, where no change in the picture takes place for a prolonged period of time, an underflow will occur in the buffer memory of the transmitting side, resulting in a difficulty that data once read is repeatedly read out. To prevent such an underflow, it has been the practice to eliminate the formation of the difference so as to supply the PCM code directly into the buffer memory for transmission to thereby permit the frame memory on the receiving side to be refreshed when the occupancy of the buffer memory is considerably reduced.
As summarized above, the PCM code from A-D conversion has been directly transmitted in order to refresh the frame memory on the receiving side. Where the refreshing technique is not employed, the significant difference signal is derived from the difference signal and quantized for transmission as a code having a reduced number of bits, which code is decoded on the receiving side to form the sum signal. However, a degradation in the image quality is caused by the quantization process or the like, with result that an area of a high image quality which is achieved by virtue of the refreshing technique appears as a streak in a restored picture, which travels vertically for successive frames as a result of a change of such area from frame to frame.
It is an object of the invention to provide an interframe coding system capable of assuring a high image quality.
It is another object of the invention to provide an interframe coding system which is insusceptible to the occurrence of an overflow from a buffer memory which is used for the purpose of smoothing the transmission bit rate, and which is also insusceptible to a degradation in the image quality.
It is a further object of the invention to provide an interframe coding system which assures a high image quality while minimizing its operation in the subsample or subline modes.
It is an additional object of the invention to provide an interframe coding system which achieves a high band compression effect and a high image quality.
It is still another object of the invention to provide an interframe coding system which refreshes a frame memory on the receiving side while preventing the occurrence of a streak.